<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>VueDemo4</title>
    <script src="js/vue.js"></script>
    <script>
        window.onload =function(){
            new Vue({
                el:'#app',
                data:{
                  msg:""
                }
            })
        }
    </script>
</head>
<body>
  <div id="app">
    用户名: <input type="text" v-model="msg">
    <br>
    <span v-text="msg"></span>
  </div>  
</body>
</html>